1. Field of the Invention
This invention generally relates to notch filters and more particularly to notch filters that have the capability of providing multiple notches in which each notch can be fixed or variable.
2. Description of Related Art
Notch filters have use in a wide variety of applications. They are particularly useful in RF applications for enhancing the reception of a weak signal or desired frequency spectrum by attenuating or rejecting a strong adjacent interfering signal or undesired frequency spectrum. If an application is characterized by a single undesired frequency spectrum, a notch filter designed for that frequency spectrum is placed in series with the signal path. However, there are a number of applications in which attenuation of two or more interfering signals is required.
In accordance with a widely accepted approach, a filter network comprises a notch filter for each notch frequency. The network cascades the individual notch filters. Thus, in theory an incoming spectra passes through each of these notch filters with each notch filter attenuating its corresponding frequency spectrum. However, the desired signals in the spectra also degrade as they pass through the successive notch filters. For example, assume an incoming spectra has the following signal spectra: EQU F.sub.SPECTRA =m.sub.1,f.sub.1 +m.sub.2,f.sub.2 +m.sub.3,f.sub.3 + . . . +m.sub.x,f.sub.x
where each m represents the magnitude and each f represents the center or nominal frequency of a frequency spectrum. Using conventional filter analyses, the series representation for each of three notch filters would be: EQU F.sub.NOTCHA =a.sub.1,f.sub.1 +a.sub.2,f.sub.2 +a.sub.3,f+ . . . +0,f.sub.NOTCHA + . . . a.sub.x, f.sub.x EQU F.sub.NOTCHB =b.sub.1,f.sub.1 +b.sub.2,f.sub.2 +b.sub.3,f+ . . . +0,f.sub.NOTCHB + . . . b.sub.x, f.sub.x EQU F.sub.NOTCHC =c.sub.1,f.sub.1 +c.sub.2,f.sub.2 +c.sub.3,f+ . . . +0,f.sub.NOTCHC + . . . c.sub.x, f.sub.x
wherein the coefficients a, b, c, . . . represent a transfer or multiplying function of each frequency component.
In a practical sense, these coefficients represent distortion and noise degradation that appears in signals at frequencies outside the undesired frequency spectra. That is, for two notch filters the output of a cascaded network will be: EQU F.sub.OUTPUT =F.sub.SPECTRA *F.sub.NOTCHA *F.sub.NOTCHB
or EQU F.sub.OUTPUT =a.sub.1 b.sub.1 m.sub.1 f.sub.1 +a.sub.2 b.sub.2 m.sub.2,f.sub.2 + . . . +0,f.sub.NOTCHA +0,F.sub.NOTCHB + . . . +a.sub.x b.sub.x m.sub.x f.sub.x
In this representation the magnitude of a frequency spectrum at a corresponding notch is "0" indicating those signals are rejected or attenuated. The remaining frequency components, however, have been degraded by the coefficients a and b. Thus it is desirable to provide some filter network that can eliminate this signal degradation of desired spectra, particularly for applications in the radio frequency range.
In some applications it can be assumed that each interfering signal has a fixed frequency. In other situations it may be necessary to adjust the notch frequency (i.e., the frequency of a frequency spectra to be attenuated). Several approaches have been suggested for accommodating multiple notches and variable frequency notches.
One approach for eliminating feedback in different and variable audio bands with multiple notch filters is disclosed in U.S. Pat. No. 4,088,834 (1978) to Thurmond. An input signal in the audio range is applied to a circuit junction that acts as a summing circuit with signals from each of a plurality of parallel, active notch filters. A first amplifier in each notch filter produces a filter output signal that is opposite in phase to the input signal. The filter output signals from all the notch filters are then applied to the summing junction at an input capacitor. The use of active filters in this application introduces a number of complexities and increases costs. Moreover, although a frequency variation is possible, the disclosed variable frequency control establishes frequency ranges and frequency adjustments. That is, the time constants for each of the plurality of RC time constant circuits must be varied thereby varying characteristics of the overall notch filter. The limitations imposed by these two characteristics therefore limits the application of the disclosed approach to signals having frequencies in the audio range.
U.S. Pat. No. 5,337,756 (1994) to Dax discloses a dynamically tunable notch filter useful in the 500 kHz to 50 MHz RF band for attenuating narcissus signals present in an electrical signal from a laser radar system. A bandpass filter, constituted by a high and low pass filters in cascade, and an amplifier connect between quadrature phase shift mixers in each of two parallel paths. As a result the RF signal is split into the two paths, modulated, passed through the bandpass filter, demodulated and combined to produce an output signal in which any narcissus signals are attenuated. Thus while the Dax patent discloses a dynamically tunable notch filter, the filter appears limited to a single notch frequency.
Therefore a need exists for a notch filter network or circuit that can handle multiple undesired frequency spectra without degrading any desired spectra. Moreover there continues to be a need for a multiple notch filter in which each notch filter can be tuned to a specific spectrum and in which each of the notch filters operates with maximum efficiency over a wide frequency range.